Using
the circuit of 40-metre band direct-conversion receiver
descr- ibed here, one can listen to amateur radio QSO
signals in CW as well as in SSB mode in the 40-metre
band. The circuit makes use of three n-channel FETs
(BFW10). The first FET (T1) performs the function of
ant./RF amplifier-cum-product detector, while the second
and third FETs (T2 and T3) together form a VFO (variable
frequency oscillator) whose output is injected into
the gate of first FET (T1) through 10pF capacitor C16.
The VFO is tuned to a frequency which differs from the
incoming CW signal frequency by about 1 kHz to produce
a beat frequency in the audio range at the output of
transformer X1, which is an audio driver transformer
of the type used in transistor radios. The audio output
from transformer X1 is connected to the input of audio
amplifier built around IC1 (TBA820M) via volume control
VR1. An audio output from the AF amplifier is connected
to an 8-ohm, 1-watt speaker. The receiver can be powered
by a 12-volt power-supply, capable of sourcing around
250mA current. Audio-output stage can be substituted
with a readymade L-plate audio output circuit used in
transistor amplifiers, if desired. The necessary data
regarding the coils used in the circuit is given in
the circuit diagram itself. |
